In HVDC power networks, switchyards are used to interconnect several sections of the network. Such switchyards may be equipped with circuit breakers which allow to disconnect one or several network sections for the purpose of maintenance, or in order to isolate a faulty section. Each section may, e.g., comprise HVDC transmission lines, converters, or any other type of HVDC equipment. For instance, a switchyard may interconnect three or more HVDC transmission lines, wherein each of the transmission lines may be individually disconnected from the two others.
Due to the quickly rising fault currents in HVDC systems, direct current (DC) circuit breakers with a sufficiently high current breaking capability are needed. Recently, DC hybrid circuit breakers capable of breaking large fault currents have been proposed, see, e.g., WO 2011/057675.
DC hybrid circuit breakers are typically based on a combination of a solid-state main breaker connected in parallel to a transfer switch, i.e., a series-connection of a solid-state auxiliary breaker and a mechanical disconnector. The principal of operation of a hybrid circuit breaker is that the main breaker, the auxiliary breaker, and the disconnector are closed during normal operation. If a trip signal is received, the hybrid circuit breaker attempts to break the current flowing through it by first opening the auxiliary breaker, thereby commutating the current flowing through the auxiliary breaker and the disconnector to the main breaker. Subsequently, the disconnector is opened for disconnecting the auxiliary breaker, and finally the main breaker is opened, resulting in a commutation of the current from the main breaker to a surge arrester connected in parallel to the main breaker. However, DC hybrid circuit breakers are comparatively expensive, mostly owing to the design of the main breaker.